This invention relates to the detection by surface ionization of particulate matter and more particularly to such detection in an instrument which combines dc current measurement with counting of particulates having distinguishing characteristics such as size.
U.S. Pat. Nos. 3,808,433 and 3,973,121 disclose apparatus for detection of very small particulate matter and macromolecules of a type which is referred to hereinafter as a surface ionization monitor for particulates.
A particulate introduced into a chamber strikes a heated surface located within the chamber and decomposes, giving to the surface its various constituents and impurities. Those constituents and impurities having ionization potentials (or electron affinities) comparable to the work function of the hot surface become surface ionized and a burst of positive or negative ions is evolved from the surface. Detection of the electrical charge of such ions registers the arrival of the particulate at the hot surface.
U.S. Pat. No. 3,808,433 is addressed primarily to the pulse counting circuitry for the detection of the individual bursts of ions and the manner in which the electrical pulse heights are related to the size of each individual particulate detected. However, with a large number of countable particulates, the instrument can become overloaded. Thus, a need exists of extend the dynamic range of surface ionization monitors for particulates to handle count rates in excess of what might be sufficient to overload pulse counting circuitry. Additionally, the simultaneous use of pulse counting of particulates with measurement of the total dc ion current, in ranges where particulate concentration do not overload the counting circuitry, permits obtaining of information about general particle size in an aerosol.